BELL. — ULTRAVIOLET COMPONENT IN ARTIFICIAL LIGHT. 



15 



by taking several preliminary swings, so as to give the thermopile 

 chance to settle into a steady state, the rate of shift of zero was kept 

 pretty steadily and the corrections were easily applied. It was also 

 necessary to photometer the arcs in the actual condition in which 

 they were under test. To this end the apparatus was set up as shown 

 in Figure 4. Here A is the arc lamp, B the thermopile, C the galva- 

 nometer, D the telescope and scale, E an adjustable rotating sector 

 disc just in front of the arc, F the quartz cells in their sliding screen 

 in front of the thermopile window, G a silvered plate glass mirror 

 which could be quickly interposed in the beam between the arc and 



© 



B 



fT^rx u//m UJ/JB j^ 



n 



\ 0/ 



\ 



X 

 .1 



Figure 4. Arrangement of radiometric apparatus. 



the thermopile so as to deflect the rays into the portable photometer 

 H, set up on the other side of the photometer room. The coefficient 

 of reflection of the mirror had previously been many times determined 

 as the mirror had been in use for photometric work. The photometer 

 was ready for use at any time simply by closing the switch on the 

 standard lamp. ^Yhen in course of a series of thermopile measure- 

 ments it was desired to test the c. p. of the lamp the disc was 

 started, the mirror swung into place and readings were then taken on 

 the portable photometer. 



